1. Field of the Invention
This invention relates to a fixing device for fixing an unfixed image on a recording material, and more particularly, to a fixing device using release oil in order to prevent offset.
2. Description of the Related Art
In an image forming apparatus, an unfixed toner image formed on a recording material is fixed by a fixing device provided within the apparatus.
The image fixing device grasps the recording material carrying the unfixed toner image by a rotating member for fixing, that is, a fixing member usually having the shape of a roller, and a pressurizing roller facing it, and fixes the toner image on the recording material.
Since such a fixing roller needs to have a degree of releasability, it has widely been practiced to coat the roller with a suitable release agent such as silicone oil.
Furthermore, it is desirable that the fixing roller have high elasticity in order to increase its fixability.
However, a material which is superior in both elasticity and releasability has not been found. Accordingly, it has been considered to provide the fixing roller with necessary functions by providing a double-layer structure in which an elastic layer made of rubber having an excellent elasticity is disposed as a lower layer and an offset prevention layer made of rubber having an excellent releasability is disposed as an upper layer.
It has been found, however, that, even if a roller having such a double-layer structure is used with a silicone oil coated thereon, there arise the following problems. That is, the releasability of the offset prevention layer does not last long, but decreases within a short period, and offset occurs after only a relatively small number of recording-material sheets are passed. Hence, the roller has a short life.
The inventors of the present invention have studied the causes of the problems and found the following facts. Since the oil absorption rate of the offset prevention layer, that is, the upper layer, is low, very little oil penetrates into the offset prevention layer even if silicone oil is coated in order to increase the releasability of the layer. Moreover, since the amount of oil absorbed in the offset prevention layer is small, much of the oil applied to the offset prevention layer is taken away by the recording material during the passage of the recording material, and the oil is rapidly exhausted. As a result, the releasing function by silicone oil disappears, and offset thereby occurs. That is, a superior releasability of the fixing roller can be obtained by a synergistic effect produced by an oil barrier formed on the surface of the offset prevention layer by an exuding effect of silicone oil from within the offset prevention layer, and by oil coated on the surface of the offset prevention layer from the outside.
In the case of using the fixing roller having the double-layer structure, the elastic layer, i.e., the lower layer, is made of oil-resistant (nonswelling) rubber which does not absorb oil, for example, fluorosilicone rubber, fluororubber, or rubber made by mixing these rubber materials with other rubber materials, such as dimethylsilicone rubber and the like, in order to prevent permeation of the oil from the offset prevention layer, i.e., the upper layer, and the swelling of the elastic layer due to the oil, as disclosed in Japanese Patent Publication Nos. 54-41332 (1979), 57-46068 (1982), and 60-21860 (1985).
Although it is possible to prevent the permeation of the oil from the offset prevention layer by using such an elastic layer, there is a problem in that the oil accumulates in the interface between the elastic layer and the offset prevention layer, and the offset prevention layer is apt easily to peel off the elastic layer. In order to solve this problem, it is intended to increase the adhesion strength between the two layers. There is a limit, however, in the increase of the adhesion strength, and hence the problem of the peeling of the offset prevention layer has not been solved yet.